1. Field of the Invention
The present invention relates to direct current power units, and more particularly, to direct current power units for efficiently charging a storage battery as a main direct current power supply using a solar battery or a fuel cell as a secondary direct current power supply and for supplying direct current power to a load from the main direct current power supply.
2. Background Art
An apparatus shown in FIG. 2 is known as a conventional direct current power unit which charges a storage battery as a main direct current power supply using a solar battery or a fuel cell as a secondary direct current power supply and supplies direct current power to a load from the main direct current power supply.
In FIG. 2, a secondary direct current power supply 1, a main direct current power supply 2, a load 3, a voltage detecting circuit 4, a switching element 5, and a diode 6 are provided. In FIG. 2, the secondary direct current power supply 1 is, for example, a solar battery 1 and the main direct current power supply 2 is, for example, a storage battery. In addition, the switching element 5 is, for example, a field effect transistor. The main direct current power supply 2 is charged by the secondary direct current power supply 1 via the switching element 5 and the diode 6. Both terminals of the voltage detecting circuit 4 are respectively connected to both terminals of the main direct current power supply 2. The voltage detecting circuit 4 detects the terminal voltage of the main direct current power supply 2. The switching element 5 is turned on or off by an output signal from the voltage detecting circuit 4 so as to prevent the overcharge of the main direct current power supply 2.
In the above-mentioned conventional direct current power unit, the turning-on or turning-off of the switching element 5 causes switching loss, and thereby the efficiency of charging the main direct current power supply 2 using the secondary direct current power supply 1 decreases.